In the field of non-impact printing, the most common types of printers have been the thermal printer and the ink jet printer. When the performance of a non-impact printer is compared with that of an impact printer, one of the problems in the non-impact machine has been the control of the printing operation. As is well-known, the impact operation depends upon the movement of impact members, such as print hammers or wires or the like, which are typically moved by means of an electromechanical system and which may, in certain applications, enable a more precise control of the impact members.
The advent of non-impact printing, as in the case of thermal printing, brought out the fact that the heating cycle must be controlled in a manner to obtain maximum repeated operations. Likewise, the control of ink jet printing, in at least one form thereof, must deal with rapid starting and stopping movement of the ink fluid from a supply of the fluid. In each case of non-impact printing, the precise control of the thermal elements and of the ink droplets is necessary to provide for both correct and high-speed printing.
In the matter of ink jet printing, it is extremely important that the control of the ink droplets be precise and accurate from the time of formation of the droplets to depositing of such droplets on paper or like record media and to make certain that a clean printed dot-matrix character results from the ink droplets. While the method of printing with ink droplets may be performed in either a continuous manner or in a demand pulse manner, the latter type method and operation is disclosed and is preferred in the present application when applying the features of the present invention. The drive means for the ink droplets is generally in the form of a well-known crystal or piezoelectric type element to provide the high-speed operation for ejecting the ink through the nozzle while allowing time between droplets for proper operation. The ink nozzle construction must be of a nature to permit fast and clean ejection of ink droplets from the print head.
In the ink jet printer, the print head structure may be a multiple nozzle type with the nozzles aligned in a vertical line and supported on a print head carriage which is caused to be moved or driven in a horizontal direction for printing in line manner.
Alternatively, the printer structure may include a plurality of equally-spaced, horizontally aligned, single nozzle print heads which are caused to be moved in back-and-forth manner to print successive lines of dots in making up the lines of characters. In this latter arrangement, the drive elements or transducers are individually supported along a line of printing.
In a still different structure, the nozzles are spaced in both horizontal and vertical directions, and the vertical distance between centers of the ink jets would equal the desired vertical distance between one dot and the next adjacent dot above or below the one dot on the paper. The horizontal distance is chosen to be as small as mechanically convenient without causing interference between the actuators, reservoirs, and feed tubes associated with the individual jets. The axes of all jets are aligned approximately parallel to each other and approximately perpendicular to the paper. Thus, if all nozzles were simultaneously actuated, a sloped or slanted row of dots would appear on the paper and showing the dots spaced horizontally and vertically. In order to produce a useful result consisting of dots arranged as characters, it is necessary to sweep the ink jet head array back and forth across the paper, and actuating each individual nozzle separately when it is properly located to lay down a dot in the desired position. A vertical row of dots is created by sequentially actuating the nozzles rather than simultaneous actuation which is the preferred practice in the more common nozzle arrangements.
A further observation in ink jet printers is that previous and current designs for drop-on-demand ink jet print heads are sensitive to the ingestion of air into or the presence of air in the supply of ink. Even a small air bubble can interrupt or fault the performance of transducers or like devices that expel ink droplets from a nozzle by means of pressure pulses created within an ink-filled chamber or channel.
Additionally, in an ink jet printer, it is important that the ink is maintained in a condition which allows the ink droplets to dry upon contact with the record media so as to avoid smearing of the ink, but at the same time, it is necessary to keep the ink droplets in a wet condition so as to prevent drying of the droplets at the print head nozzle.
Certain printing inks have a slow drying characteristic and have been used in print heads along with the use of absorbent type paper so that the paper can take care of part of the problem of the slow drying ink. A disadvantage of the use of such slow drying ink and absorbent paper is that the behavior of the ink and paper causes irregular dot patterns and distorted characters.
Certain other printing inks having quick drying characteristics have been used on high quality papers and wherein these inks include organic solvents, such as ketone or alcohol along with water and a dye. However, such printing inks having an organic solvent base tend to dry or evaporate at the print head nozzle or within the print head itself or even within the ink supply system. It has been found that in the case of printing inks which consist primarily of water and ethylene glycol that the water has a tendency to evaporate or to slowly permeate through the wall of the ink supply conduit or channel.
Since it is common practice to provide a polyvinyl chloride tubing for carrying the printing ink from a reservoir or like supply to the print head, it is advantageous to use a printing ink having a formulation whereby permeation of the water through the wall of the tubing has minimum effect on the operation of the ink jets or nozzles. One area of concern has been the effect of a change in the condition of the printing ink in an ink jet printer which has been sitting idle for an extended period of time. Several suggestions for minimizing evaporation of the water have been the use of a printing ink having a low vapor pressure, a flexible and substantially water impermeable tube or conduit, and a tube or conduit having a relatively large wall thickness.
Representative documentation in the area of ink jet printing and conditioning of the ink includes U.S. Pat. No. 4,234,885, issued to G. W. Arway on Nov. 18, 1980, which discloses a system for controlling the flow of pressurized liquid to a print head through an elastic conduit and including an outer conduit to form a pressurizable jacket surrounding the inner conduit and having pressure control means to prevent dribbling or drooling of liquid at the print head immediately following shut off.